1. Field of the Invention
Embodiments of the present invention relate to a charged particle beam apparatus and, more particularly, to an arrangement and a method to compensate for variations in the tip location, especially tip vibrations of an emitter tip. More specifically, embodiments described herein relate to a charged particle beam apparatus as well as to a method of compensating variations in an emitter location of a charged particle beam apparatus.
2. Description of the Related Art
Charged particle beam apparatuses are used in a plurality of industrial fields, including, but not limited to, high resolution imaging and processing of samples, inspection of semiconductor devices during manufacturing, exposure systems for lithography, detecting devices and testing systems. There is a high demand for structuring, testing and inspecting specimens within the micrometer and nanometer scale. Micrometer and nanometer scale process control, inspection, or structuring is often done with charged particle beams, (e.g., electron beams). Charged particle beams offer superior spatial resolution compared to, for example, photon beams due to their short wavelengths.
Although the prior art and embodiments of the present invention will be described in the following with reference to electrons, electron beams, electron emitters, or electron microscopes, those skilled in the art will understand that the explanations are also true for other charged particles, such as ions, ion beams, ion emitters, etc.
The first step in the process of creating images in any electron microscope is the production of an electron beam. The electron beam is generated in a device often called an electron gun. Three major types of electron guns are used in electron microscopes: tungsten-hairpin filament guns, lanthanum-hexaboride (LaB6) guns, and field-emission guns. Field-emission guns offer several advantages over tungsten-hairpin filament guns or LaB6 guns. First, the brightness may be up to a thousand times greater than that of a tungsten gun. Second, the electrons are emitted from a point more narrow than that in the other sources. Thus, superior resolution is achieved by field-emission guns compared to tungsten or LaB6 guns. Furthermore, the energy spread of the emitted electrons is only about one-tenth that of the tungsten-hairpin gun and one-fifth that of the LaB6 gun. Finally, the field-emission gun has a very long life, up to a hundred times that of a tungsten gun. For these reasons, the field-emission gun is a good choice for a number of applications.
The typical construction of a conventional electron emitter, such as a thermal field-emission (TFE) gun, a cold field-emission (CFE) gun, or a field-assisted photocathode, is shown in FIGS. 9a to 9c. In FIG. 9a, the emitter assembly is mounted on an insulating ceramic base 1, which is normally a ceramic socket. A hairpin wire (support) 3 is attached to two metal support pins 2. The hairpin wire 3, which is made typically out of tungsten, can also be used as a heater in cases where the emitter requires heat for normal operation, for cleaning, for processing or for other reasons. The emitter 4 is supported by a supporting member formed by the base, the support pins and the hairpin wire (filament). Typically, the bent tungsten wire 3 is attached to support pins 2 by spot welding. The rear end 2b of the support pins are used as connection terminals. A very finely curved sharp tungsten tip serves as the emitter tip (particle beam source) 4 and is attached to the bent tungsten wire 3. Typically, the emitter tip 4a is attached to the heating filament 3 by spot welding.
However, the conventional field-emission gun shown in FIGS. 9a to 9c suffers, for example, from mechanical vibration of the emitter. Mechanical vibrations of the emitter tip significantly limit the achievable resolution. This applies to many corpuscular beam systems, but in particular to scanning particle beam systems.
The problem of mechanical vibration will be explained with reference to FIGS. 9d and 9e. FIG. 9d shows a first vibrational mode of the conventional field-emission gun shown in FIGS. 9a to 9c. In this first vibrational mode, the emitter tip 4a undergoes a displacement in the x-direction. However, the emitter configuration is stiff in the x-direction so that such a displacement in x-direction corresponds to a higher order vibrational excitation which may even include torsion movements of the heating filament 3. Accordingly, such a high order vibrational mode has a very high eigenfrequency and is strongly damped. Therefore, this first vibrational mode has only a very small amplitude and, therefore, has not yet been observed in experiments.
FIG. 9e shows a second vibrational mode of the conventional field-emission gun shown in FIGS. 9a to 9c. In this second vibrational mode, the emitter tip 4a undergoes a displacement in the y-direction. This displacement in the y-direction is caused by bending of the heating filament 3. While being stiff in the x-direction, the emitter configuration is not very stiff in the y-direction so that a bending movement of the heating filament 3 in the y-direction corresponds to a lower order vibrational mode. Typically, this second vibrational mode of the emitter has an eigenfrequency of about 2 kHz. Furthermore, the damping is not very strong so that the second vibrational mode has a considerable amplitude. In fact, this amplitude may be so large, (e.g., within the nanometer range) that it can be observed in an experiment. Consequently, the displacement of the emitter tip 4a in the y-direction limits the resolution of some electron microscopes, especially for hairpin sources with an emitter needle welded on top of the hairpin, which are used in many applications like scanning electron microscopes (SEMs), focused ion beams (FIBs), writing and modification tools.
In particular, the second vibrational mode can be introduced by vibrations of the system or acoustic noise. The frequencies of these vibrations are in the kHz regime, and amplitudes of several nanometers can occur. The tip vibrations become resolution-limiting in particle beam system with particle beam sources of small (virtual) size. Examples are cold field emitters (CFEs) in electron-beam technology, which have a virtual sources size of about 3 nm. Ion beam technology sources with small effective diameters are also known.
In the past it has been suggested to stabilize the emitter tip by adding an additional filament, that is a third wire, which may then be arranged, for example, in an angle of 90° to the filament shown in FIGS. 9a to 9e. Thereby, particularly the second vibrational mode is intended to be reduced. Such a device can increase the stability or stiffness of the arrangement to a certain degree. Nevertheless, when the tip is heated through the wire, an arrangement having more than two connections to the terminal positioned in one plane may introduce a drift due to deformation of the wires. Further, it is still difficult to guarantee a very high stability. For high resolution applications, with a resolution of 1 nanometer or below, stability of 1 nm or below would need to be guaranteed.